Wavelength division multiplexing of optical signals is finding widespread use in various fields, including especially for data transmission and other telecommunication applications. The use of wavelength division multiplexing in fiber-optic systems has gained interest as a feasible method of increasing data transfer capacity of a fiber-optic line and/or other waveguide. In particular, wavelength division multiplexing can increase capacity of a fiber-optic trunk line at substantially lower cost than the laying of additional new fiber-optic lines. Wavelength division multiplexing allows multiple signals at different wavelengths to be carried simultaneously by a fiber-optic line or other waveguide.
The increase in carrying capacity of a fiber-optic line can be approximately linearly proportional to the number of multiplexed channels. That is, for example, a fiber-optic system employing 16 channel wavelength division multiplexing has approximately sixteen times the carrying capacity or throughput at a given bit transfer rate as the same system not employing wavelength division multiplexing. Presently preferred wavelength bands for fiber-optic transmission media include those centered at 1.3 m and 1.55 m. The latter is especially preferred because of its minimal absorption and the commercial availability of erbium doped fiber amplifiers. The useful bandwidth is approximately 10 to 40 nm, depending on application. Wavelength division multiplexing can separate this bandwidth into multiple channels. Ideally, the 1.55 m wavelength band, for example, would be divided into multiple discreet channels, such as 4, 8, 16 or even as many as 32 or more channels, through a technique referred to as dense channel wavelength division multiplexing, as a low cost method of substantially increasing a waveguide's signal carrying capacity, such as long-haul telecommunication capacity over existing fiber-optic transmission lines. The International Telephony Union (ITU) Grid provides standard center wavelengths for channels in the 1.55 m wavelength band, at 100 Ghz spacing (approximately 0.8 nm). Wavelength division multiplexing may be used to supply telephony and data transmission and, more and more in the future, such services as video-on-demand and other existing or planned multimedia, interactive services. Techniques and devices are required, however, for multiplexing the different discreet carrier wavelengths. That is, the individual optic signals must be combined onto a common fiber-optical waveguide and then later separated again into the individual signals or channels at the opposite end of the fiber-optic cable. Thus, the ability to effectively combine and then separate individual channels (or wavelength bands) on a fiber-optic trunk line or other optical signal source is of growing importance to fiber-optic telecommunications and other fields.
Known devices for this purpose have employed, for example, diffraction gratings, prisms and various types of fixed or tunable filters. Gratings and prisms typically require complicated and bulky alignment systems and have been found to provide poor efficiency and poor stability under changing ambient conditions. Fixed wavelength filters, such as interference coatings, can be made substantially more stable. In this regard, quality interference coatings of metal oxide materials, such as niobia and silica, can be produced by commercially known plasma deposition techniques, such as ion assisted electron beam evaporation, ion beam sputtering, and reactive magnetron sputtering, e.g., as disclosed in U.S. Pat. No. 4,851,095 to Scobey et al and U.S. Pat. No. 5,525,199 to Scobey. Such coating methods can produce interference cavity filters formed of stacked dielectric optical coatings which are advantageously dense and stable, with low film scatter and low absorption, as well as low sensitivity to temperature changes and ambient humidity.
Optical multiplexing devices are known for combining the multiple channel signals at one end of a trunk line and for separating out the individual signals at the opposite end of the trunk line. That is, multiplexing here refers to adding channels, removing channels or both. For simplicity of explanation, only the demultiplexing functionality is described here in detail, since those skilled in the art will readily understand the correlative multiplexing functionality. That is, those skilled in the art will recognize how the same device can be employed in reverse. The term "multiplexing" will be used here to refer to both the combining and separating of channels. The term "trunk line" is used here to refer to any fiber-optic or other waveguide carrying a multi-channel optical signal, that is, a signal comprising multiple wavelength sub-ranges multiplexed together on the trunk line. It is known to optically couple a trunk line carrying multiple channels to a common port of a wavelength division multiplexer ("WDM"--this term is used here to mean devices which combine signals, separate signals or both). Such WDM common port is, in turn, optically coupled within the WDM to multiple channel ports. Associated with each channel port is an interference filter or the like which is substantially transparent to the wavelength band of that particular channel. Thus, signals having the wavelength assigned to a particular channel are passed by the WDM through its respective channel port to and/or from the individual waveguide for that channel.
Interference filters of the Fabry-Perot type, which are preferred in various multiplexing applications, typically transmit only a single wavelength or range of wavelengths. Multiple filter units can be used together in a WDM, e.g., on a common parallelogram prism or other optical block. Multiple optical filters are joined together, for example, in the multiplexing device of U.K. patent application GB 2,014,752A to separate light of different wavelengths transmitted down a common optical waveguide. At least two transmission filters, each of which transmits light of a different predetermined wavelength and reflects light of other wavelengths, are attached adjacent each other to a transparent substrate. The optical filters are arranged so that an optical beam is partially transmitted and partially reflected by each optical filter in turn, producing a zigzag light path. Light of a particular wavelength is subtracted or added at each filter. Similarly, in the multiplexing device of European patent application No. 85102054.5 to Oki Electric Industry Co., Ltd., a so-called hybrid optical wavelength division multiplexer-demultiplexer is suggested, wherein multiple separate interference filters of different transmissivities are applied to the side surfaces of a glass block. A somewhat related approach is suggested in U.S. Pat. No. 5,005,935 to Kunikani et al, wherein a wavelength division multiplexing optical transmission system for use in bi-directional optical fiber communications between a central telephone exchange and a remote subscriber employs multiple separately located multiplexers each having separate filter elements applied to various surfaces of a parallelogram prism.
In addition to multiplexing signals at opposite ends of a trunk line, systems employing wavelength division multiplexing have been evolving more complicated architectures, employing, for example, add/drop optical multiplexing devices for removing and/or injecting a single channel at any point along a trunk line. Filter devices for multiplexing a single wavelength subrange, and the use of a series of such devices for multiplexing multiple wavelength subranges in sequence, are shown for example, in U.S. Pat. No. 4,768,849 to Hicks, Jr. In that patent multiple filter taps, each employing dielectric filter mirrors and lenses for removing (or adding) one channel from a multi-channel trunk line, are shown in use singly and in arrays for removing a series of channels. It has been suggested to use a single narrowband cavity filter as an add/drop optical multiplexing device. As shown in FIG. 1, a previously known add/drop optical multiplexing device employs a filter element 10 comprising a narrowband cavity filter 11 carried on a suitable optical substrate 12. The signal from a trunk line 14 carrying multiplexed channels 1-n is passed through a collimator 16 to the filter element 10. The signal corresponding to channel m is in-band of narrowband cavity filter 11 and, accordingly, passes through filter element 10 to collimator 18.
Thus, the device serves to drop channel m from the multiplexed signal. Collimator 18 is optically coupled to branch line 20, comprising a fiber-optic or other waveguide. Alternatively, channel m can be passed from filter element 10 to an optical detector or other destination. The remainder of channels 1 through n are not in-band of cavity filter 11 and, accordingly, are reflected by cavity filter 11 to collimator 22 of a common pass through which is optically coupled to a downstream portion 15 of trunk line 14. A branch feed line 24 optically coupled to collimator 26, or other optical signal source, serves to add channel m' to the multiplexed signal. It passes an optical signal 28 for channel m' to filter element 10. Channel m' employs the same wavelength sub-range as channel m, but has a different "payload" or information. The downstream portion 15 of the trunk line 14 thus carries a multiplexed signal comprising channels I through n, which includes a signal m' in the wavelength sub-range made available by dropped channel m.
A problem encountered in the use of previously known devices of the type shown in FIG. 1, is that a residual portion of the signal of channel m reflects from filter 11 and is reimaged into collimator 22. This light then introduces crosstalk to channel m', which is by definition the same wavelength as the residual reflection. A typical filter will reflect between 2.5% to 10% (-16 to -10 dB) of the light, as shown in the graph of FIG. 2. As shown there, the depth of the notch function of an interference filter (measured at the worst performance point, that is, the highest reflectance point across the in-band range) is limited in general to -16 dB over the in-band region.
In contrast, current specifications for a typical fiber-optic telecommunication system may call for a channel drop efficiency of at least -30 dB to -40 dB (corresponding to residual reflection of no more than 0.1% to 0.01%). It is not commercially practicable to produce a -30 dB or -40 dB filter element employing a single filter as in the devices described above, using currently available materials and manufacturing techniques for narrowband cavity filters, especially for tight channel spacings such as 200 Ghz or even 100 Ghz, as are presently proposed for fiber-optic telecommunication systems. For filters used in transmission, better isolation can be achieved by using a filter with a higher number of cavities. However, as the number of cavities increases and the complexity of the coating increases, typically, the notch depth (see FIG. 2) becomes less (that is, there is greater in-band reflectance) due to increased residual reflection. That is, the greater the number of cavities, the steeper the slopes and the higher the figure of merit in transmission, but typically the residual reflection of the in-band region increase, which reduces the isolation of the add/drop multiplexing device. The figure of merit ("FOM") as used here, unless another meaning is clear from the context, means the ratio of the width of the in-band reflectance curve at -30 dB to the width of the in-band reflectance curve at -1 dB ("FOM 30/1)", or at -0.5 dB ("FOM 30/0.5") or at -0.25 dB ("FOM 30/0.25"). A higher FOM is advantageous, as it corresponds to lower reflectance to the pass-through port of the signal extracted at the drop channel port of the add/drop multiplexing device.
It is an object of the present invention to provide an add/drop optical multiplexing device which provides excellent signal isolation employing filter elements which can be produced using currently commercially available manufacturing materials and techniques. It is an object of at least preferred embodiments of the invention to provide a fiber-optic telecommunication system employing one or more such add/drop optical multiplexing devices. Additional objects and advantages of the invention will become apparent from the following disclosure of the invention, including detailed description of certain preferred embodiments.